In this week's "Ask a Physicist," we'll explore an awesome mash-up of exoplanets, supercivilizations, and dark matter, and try to figure out whether the "missing mass" in the universe is really advanced aliens with a cloaking device.

It's not often that I get a question that is straightforward, reasonable and terrifying as hell. Reader Marc Unger tried to make me question my place in the cosmos with the following puzzler:

Given the tremendous numbers of planets that we are starting to observe in the galaxy: isn't it possible that the "missing mass" of the galaxy (and the universe) is actually because alien species have built a huge number of Dyson spheres?

I feel kind of silly explaining the concept of a Dyson Sphere to io9 readers, since I'm sure you all have your own under construction, but I'll do so for those readers who are more on the fantasy side of the sci-fi spectrum.

In 1960, the physicist Freeman Dyson proposed the idea that a supercivilization might build a giant sphere around a star that was so large that it had a radius approximately equal to the distance from the earth to the sun. Since all of the starlight would be absorbed by the sphere, the entire interior surface could be kept at room temperature. If you played your cards right, you'd have about a billion times the arable land on the interior of the sphere than we do on the surface of the earth.

Oh sure, there are little details that might make living in a Dyson Sphere kind of a pain. For instance, if you wanted to use the whole thing there's no way to spin it in such a way to give everyone a nice earth-like artificial gravity. Also, if you wanted to experience night, you'd basically have to go underground (which is to say, outerground), but isn't that a small price to pay for allowing the population to grow to quadrillions? And yes, I'm aware, that there are variants like the "Dyson Swarm" which don't have these problems, but they wouldn't hide their stars, so we're going to ignore them.

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Could we be surrounded by Kardashev Type 2 civilizations? In Marc's picture, the Galaxy is so filled with hideous intelligence that it's really just a matter of time before we're invaded, assimilated, and, at best, turned into alien sex slaves. There's also the small fact that virtually all of the stars — perhaps tens of billions of them in our Galaxy alone — would be just out of sight, neatly tying together the question of Dark Matter and extraterrestrials.

I've written a fair amount about the evidence for dark matter, so I won't rehash all of it now. If you'd like the broad-brush strokes, the idea is that everything from the scale of galaxies on up seem to be "missing" about 85% of their mass. It doesn't seem to be in the form of stars or gas, but could it be in the form of Dyson Spheres?

Dyson introduced his Spheres not so much as a "how to become a supercivilization" instruction manual as a "how to find a supercivilization" manual. Everything that takes in energy ultimately re-radiates it. This is true, on average, of the earth, for instance, and if it weren't we'd heat up at an alarming rate. Likewise, you absorb light and take in fuel and as a result, you heat up and glow in the dark, though not in the wavelength range that our eyes are sensitive to. You glow in the infrared rather than the visible. This is how night-vision goggles work.

It's also how the Spitzer Space Telescope, which went up in 2003, works. Dyson knew that his eponymous spheres would be heated to (roughly) room temperature, just by assuming that all creatures are as fond of liquid water as we are. At those temperatures, the Dyson Spheres should all be radiating at approximately 10 micrometers, right in the middle of the range that Spitzer is sensitive to. Even if there were only a few Dyson Spheres in our Galaxy and Spitzer would still be able to pick them out. After all, they're still radiating the energy of an entire star.

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2) Gravity leaves a trace.

One of the reasons that I like this question is that it goes far beyond the specifics of Dyson Spheres, and gets at something much more general. Most cosmologists think that Dark Matter is some sort of particle — just one we haven't yet discovered. But think about it, couldn't there be some giant spheres: maybe Dyson Spheres, maybe ordinary black holes, flying around our Galaxy? If they're dark, we'd never see them.

These objects have a name: MAssive Compact Halo Objects (or, their juvenile acronym MACHOs). Fortunately, we don't need to detect MACHOs from their light alone. In 1936, Einstein predicted an effect of general relativity known as "microlensing." Microlensing uses the gravitational focusing power of gravity to temporarily brighten a star when it passes behind a massive object — any massive object. This would include anything: ordinary stars, black holes, brown dwarves, and even Dyson Spheres. Einstein wasn't terribly optimistic about actually detecting this since only about 1 in a million stars gets microlensed. Fortunately, modern observatories are able to observe millions of stars simultaneously, and we get to observe lots and lots of microlensing events.

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What's the verdict? After more than a decade of observing, the best estimates indicate that at most something like 10% of the mass our Galaxy could be in the form of MACHOs, and possibly much less. Remember, also that this includes pretty boring stuff like Brown Dwarves (stars that are too puny to ignite and form proper stars) and the like.

3) The Big Bang Limits our Chemistry Set

The problem with Dyson Spheres (and any other MACHOs) is that they're ultimately made from boring old atoms, and it turns out, there's just not enough atoms out there. In many ways, physics in the early universe was way simpler than it is now. With a few relatively simple assumptions, we can make a lot of predictions. This may perhaps be a question for another day, but we're able to get extraordinarily accurate measurements of the baryon (aka ordinary matter) to Dark Matter ratio from the Cosmic Microwave Background, and completely independently get the same ratio by measuring the amount of various light elements in the universe today.

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For instance, suppose we assume that there's about 5 times as much Dark Matter as baryonic matter (the number that we get from the Cosmic Microwave Background estimates). From that simple input, we can predict all sorts of things, including the fraction of atoms that are in the form of helium, the fraction that are in the form of deuterium (a sort of super hydrogen with a neutron as well as a proton), and so on, for the other light elements. These numbers are dead-on with the ratios that we actually observe. Increase the fraction of "ordinary matter" (or equivalently decrease the amount of dark matter) by even a few percent and a bunch of observations suddenly become totally inconsistent. What's more, these estimates of the baryons are pretty much in line with the numbers that we get by just counting up the mass in gas and stars in galaxies.

The dark matter can't be made of atoms because the big bang tells us, very precisely, what the density of atoms are in the universe.

If you're making Dyson Spheres, the problem becomes even worse. The big bang only produced the very lightest elements, but to make technology you're going to need iron and copper and silicon and the like. To put in things in perspective, even if the Dyson Spheres were only a few feet thick, you'd still need as much material as the entire mass of Saturn (which, if you didn't know, is mostly made of hydrogen). In reality, you'd need your spheres to be much, much larger than that. Heavy elements didn't get made until the most massive stars came along and built them by going boom! in a supernova explosion. While there's certainly enough material in the universe to build a few Dyson Spheres, there's no way that they (and the stars that they're hiding) can make up the bulk of what's out there. Even if the spheres themselves were extremely lightweight, and made up less than 1% of the total mass of their stars, that's way more of the metals than actually exist in the universe.

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So even though we're not surrounded by aliens, we are surrounded by unknown dark matter particles, which might be infinitely creepier.